Negotiators from Iran and six world powers are holding fresh talks in the long-running standoff over the country's nuclear program.

The meetings on Tuesday and Wednesday in Geneva come amid what European Union foreign policy chief Catherine Ashton called "cautious optimism". These talks are the first since Iran elected a new president.

The United States, Russia, China, France, Britain and Germany want Iran to allay concerns that it is developing nuclear weapons while Iran says its program is peaceful and seeks relief from international sanctions.

Iranian President Hassan Rouhani has promised to lead a diplomatic effort to get the sanctions eased, but officials from the so-called P5+1 nations have expressed the need for Iran to prove its sincerity through concrete steps before that will happen.

Israeli Prime Minister Benjamin Netanyahu's security cabinet warned the world powers against making any partial deal with Iran, saying they must reject any agreement "that leaves Iran with the capability to develop nuclear weapons."

In Washington, a bipartisan group of leading U.S. senators said it is open to suspending the implementation of new sanctions on Iran but only if Tehran takes significant steps to slow its nuclear program.

In a letter sent to President Barack Obama last week and released Monday, the 10 senators said the U.S. and other countries should consider a "suspension-for-suspension" agreement, in which Iran suspends uranium enrichment and Washington suspends the implementation of new sanctions.

However, Iran is not expected to offer to suspend enrichment during the talks.

A senior U.S. official, speaking on condition of anonymity, told reporters that "no one should expect a breakthrough overnight."

The official said incremental steps are needed to build confidence and "put time on the clock" so negotiators can work through complex political and technical issues.

In previous rounds of negotiations, world powers called for Iran to give up its existing stockpile of uranium enriched to 20 percent purity and send it abroad. Uranium of that purity is a short technical step away from being converted to weapons-grade material.